The present invention relates to an optical pickup that is provided with a plurality of light sources emitting light beams with different wavelengths (for example, semiconductor lasers), an optical information recording/reproducing apparatus using the same, and a phase variable wave plate used in the pickup and the apparatus.
In 1996, a DVD (digital versatile disk) system having a recording capacity of 4.7 GB was developed by using an AlGaInP red semiconductor laser (a wavelength of approximately 650 nm). A conventional CD (compact disk) system had used an AlGaAs near infrared semiconductor laser (a wavelength of approximately 780 nm) and had a recording capacity of 650 MB.
There are many points differentiating the DVD system from the CD system. One of them is the base material thickness of an optical disk used. Specifically, the base material thickness of the optical disk is 1.2 mm in the CD system, while it is 0.6 mm in the DVD system. Accordingly, in order to obtain a compatibility with the CD system, various methods are being suggested in the DVD system.
One of them is a configuration using a bifocal lens (for example, see xe2x80x9cOptical Review,xe2x80x9d Vol. 1, No. 1, pages 27-29 (1997)).
The bifocal lens is a lens in which concentric circular hologram elements are formed on an objective lens with a numerical aperture NA of 0.6 designed for the 650 nm wavelength. This bifocal lens can separate light that is focused without aberration on a CD with a base material thickness of 1.2 mm by using a+first order light of the hologram element from light that is focused without aberration on a DVD with a base material thickness of 0.6 mm by using the usual objective lens (a zeroth-order light of the hologram element), thereby achieving the compatibility between the CD and the DVD.
However, although the optical system using the bifocal lens realizes the compatibility with the CD, it cannot yet achieve that with a CD-R. This is because a sufficient reproducing signal cannot be obtained due to the rather poor reflective properties of the CD-R in the red region. Thus, an optical pickup having two integrated units 125 and 126 (wavelengths of 650 nm and 780 nm) as shown in FIG. 21 is suggested.
In the configuration shown in FIG. 21, a laser beam with a wavelength of 650 nm emitted from the integrated unit 125 for DVD passes through a wavelength branching prism 127, a polarizing hologram 128 (diffraction gratings are formed on a LiNbO3 substrate by a proton exchange) and a wave plate 129 (a (5/4) xcex plate for 650 nm wavelength), and then is focused on an optical disk (DVD-ROM) 131 by an objective lens 132. The light reflected by the optical disk 131 enters the wave plate 129 by which a polarization direction thereof is rotated by 90xc2x0 from that of an incident light, is diffracted by the polarizing hologram 128 and is imaged on a photo detector (PD) in the integrated unit 125 for DVD. In this optical detection system, the focusing direction is controlled by a SSD (spot size detection) method, and the tracking direction is controlled by a phase difference detection method.
On the other hand, a laser beam with a wavelength of 780 nm emitted from the integrated unit 126 for CD passes through a plastic hologram element 126b with a narrow pitch and is reflected by the wavelength branching prism 127. Then, as the laser beam with a wavelength of 650 nm from the integrated unit 125 for DVD, it passes through the polarizing hologram 128 and the wave plate 129, and is focused on an optical disk (CD or CD-R) 130 by the objective lens 132. The light reflected by the optical disk 130 passes through the wave plate 129 and the polarizing hologram 128 again. In this case, since the wave plate 129 functions as a xcex plate for 780 nm wavelength, the polarization direction is maintained and not diffracted by the polarizing hologram 128. The light reflected by the wavelength branching prism 127 and then diffracted by the plastic hologram element 126b is imaged on a photo detector (PD) in the integrated unit 126 for CD. In this optical detection system, the focusing direction is controlled by the SSD method, and the tracking direction is controlled by a three-beam method.
The objective lens 132 is designed so that the 780 nm wavelength light causes small aberration for the optical disk (CD or CD-R) 130 with a base material thickness of 1.2 mm and the 650 nm wavelength light causes small aberration for the optical disk (DVD-ROM) 131 with a base material thickness of 0.6 mm.
With the optical pickup having the above configuration, the optical disk (CD or CD-R) 130 with a base material thickness of 1.2 mm is reproduced by the laser beam with a wavelength of 780 nm emitted from the integrated unit 126 for CD, and the optical disk (DVD-ROM) 131 with a base material thickness of 0.6 mm is reproduced by the laser beam with a wavelength of 650 nm emitted from the integrated unit 125 for DVD, thereby obtaining excellent reproducing properties.
The recording capacity of a current DVD is 4.7 GB, and approximately two hours of NTSC (National Television System Commitee standard) broadcast data can be recorded. However, in order to develop media for image data of a high-vision or a high-definition (generally referred to as xe2x80x9cHDxe2x80x9d in the following), it is essential to further improve the recording density of the optical disks.
As means for improving the recording density of the optical disks, (1) changing a light source so as to produce a shorter wavelength and (2) increasing the numerical aperture NA of the objective lens can be considered. However, making the numerical aperture NA of the objective lens larger than the current value of 0.6 is difficult both from the viewpoints of margins on systems and of compatibility with the CDs and the DVDs.
On the other hand, the light source can be changed so as to produce a shorter wavelength by using a second harmonic generation (SHG) technique of a near infrared semiconductor laser or by using a GaN semiconductor laser. The use of blue light with a wavelength of approximately 400 nm can improve the recording density by approximately 2.3 times over the current DVD. In the following, the DVD that is obtained as above will be referred to as xe2x80x9cHD-DVDxe2x80x9d.
Also in the era of HD-DVDs using blue light, it is important to obtain compatibility with the DVDs and the CDs. In line with the CD-R, a pigment-type DVD-R is currently being developed. However, the reflective properties of the CD-Rs and the DVD-Rs deteriorate in the blue region. Therefore, in order to achieve compatibility, an optical pickup that is provided with coherent light sources respectively emitting lights in three wavelength regions of blue region, red region and infrared region is necessary.
However, since the optical pickup that is configured with the coherent light sources of multiple wavelengths necessitates many optical components, it is difficult to design an optical pickup that can be mass-produced on a practical level. For example, problems described below are caused.
(1) Due to the increase of the number of the optical components, higher precision in aberration of respective optical components becomes necessary, and integration thereof becomes difficult, leading to the difficulty in designing a small-size (thin) optical pickup, and
(2) Since it is necessary to simultaneously achieve the optical detection systems corresponding to respective lights of multiple wavelengths, the configuration of a 1/4 wave plate becomes complex when using a polarizing hologram element and a polarization branching element in the optical detection systems.
It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a small-size optical pickup that, with a simple configuration, achieves compatibility between multiple types of optical disks and stable signal detection even when using a polarizing optical detection system, an optical information recording/reproducing apparatus using the same, and a phase variable wave plate used in the pickup and the apparatus.
In order to achieve the object mentioned above, the first configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes two coherent light sources respectively emitting light beams with different wavelengths (xcex1 less than xcex1), a light coupling member, and a wave plate. The light beams emitted from the two coherent light sources are coupled by the light coupling member so as to be propagated on the same optical axis, pass through the wave plate, and are then focused on an optical disk, and a retardation amount xcex of the wave plate is in the range 3/4xc2x7xcex2 less than xcex less than 5/4xc2x7xcex1. The first configuration can provide the optical pickup in which a simple configuration can achieve a compatibility with many types of optical disks and a stable signal detection even when using a polarizing optical detection system.
The second configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes two coherent light sources respectively emitting light beams with different wavelengths (xcex1 less than xcex2), a light coupling member, and a wave plate. The light beams emitted from the two coherent light sources are coupled by the light coupling member so as to be propagated on the same optical axis, pass through the wave plate, and then are focused on an optical disk, and a retardation amount xcex of the wave plate is substantially an uneven integer multiple of xcex1/4 and substantially an uneven integer multiple of xcex2/4 . The second configuration can provide an optical pickup in which a simple configuration can realize a compatibility with many types of optical disks and a stable signal detection even when using a polarizing optical detection system.
In the second configuration of the optical pickup according to the present invention, it is preferable that the wave plate is made of a birefringent material with large wavelength dispersion, and the retardation amount xcex of the wave plate is (2n+3) times xcex1/4 for the light beam with the wavelength xcex2 and is (2n+1) times xcex2/4 for the light beam with the wavelength xcex2, wherein n=0, 1, 2, . . . . With this preferred example, a wave plate that, for example, converts a linear polarization to a circular polarization for both red light (wavelength of 690 nm) and blue light (wavelength of 380 nm) can be manufactured.
In the first or second configuration of the optical pickup according to the present invention, it is preferable that the wavelengths of the two coherent light sources are in the ranges 370 nm less than xcex1 less than 430 nm and 635 nm less than xcex2 less than 690 nm.
The third configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes n coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . .  less than xcexn) wherein n greater than 2, and (nxe2x88x921) light coupling members. The light beam with the wavelength xcex1 passes through all (nxe2x88x921) light coupling members, and the light beams with other wavelengths are respectively reflected by the light coupling members that are disposed corresponding to the coherent light sources emitting the light beams, so that all the light beams with different the wavelengths are coupled so as to be propagated on the same optical axis. The third configuration of the optical pickup can achieve an optical pickup with small aberration for every wavelength light.
The fourth configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, a light coupling member, and a phase variable wave plate. At least one of the optical detection systems includes a polarization branching member. The light beams emitted from the plurality of coherent light sources are coupled by the light coupling member so as to be propagated on the same optical axis, pass through the phase variable wave plate and are then guided to the optical disk. A principal crystal axis of the phase variable wave plate and a radial direction of the optical disk are in parallel or perpendicular to each other. A polarization direction of the coupled light beam is inclined by 45xc2x0 with respect to the radial direction of the optical disk. With the fourth configuration of the optical pickup, a single phase variable wave plate can have functions both as a 1/4 wave plate and of compensating for a birefringence amount generated in the optical disk.
In the first to fourth configurations of the optical pickup according to the present invention, it is preferable that the light coupling member is a dielectric multi-layer film mirror. This preferred example can improve the efficiency of light utilization.
The fifth configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes a plurality of coherent light sources that are mounted on the same submount and emit light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ), and a collimator lens converting all the light beams with the different wavelengths into parallel beams. The fifth configuration of this optical pickup has one collimator lens, which converts the light beams emitted from the light sources into parallel beams, thereby simplifying the adjustment of the collimator lens.
In the fifth configuration of the optical pickup according to the present invention, it is preferable that the coherent light source emitting the light beam with the wavelength xcex1 is mounted on the submount in a position corresponding to a center of an optical axis of the collimator lens. This preferred example can achieve an optical pickup with small aberration for every wavelength light.
The sixth configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. At least one of the optical detection systems includes a polarization branching member, and the phase variable wave plate is located behind the polarization branching member. With the sixth configuration of this optical pickup, even when at least one of the optical detection systems includes a polarization branching member, a constant light quantity guided onto the photo detectors can be maintained.
The seventh configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. The phase variable wave plate includes two liquid crystal layers with different wavelength dispersion relationships of refractive index, the two liquid crystal layers are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1. The seventh configuration of this optical pickup can achieve the phase variable wave plate that functions as substantially a 1/4 wave plate for every wavelength light and a stable signal detection even when using a polarizing optical detection system. Also, since adjusting a voltage applied to the liquid crystal layer enables a fine adjustment of the properties, the phase variable wave plate can function as the 1/4 wave plate in a stable manner even when an environmental temperature changes.
In the fourth or sixth configuration of the optical pickup according to the present invention, it is preferable that the phase variable wave plate includes two liquid crystal layers with different wavelength dispersion relationships of refractive index, the two liquid crystal layers are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1.
The eighth configuration of the optical pickup according to the present invention is characterized in that the optical pickup includes a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. The phase variable wave plate includes a combination of a liquid crystal layer and a film with different wavelength dispersion relationships of refractive index, the liquid crystal layer and the film are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1. The eighth configuration of this optical pickup can achieve the phase variable wave plate that functions as substantially a 1/4 wave plate for every wavelength light and a stable signal detection even when using a polarizing optical detection system. Also, since adjusting a voltage applied to the liquid crystal layer enables a fine adjustment of the properties, the phase variable wave plate can function as the 1/4 wave plate in a stable manner even when an environmental temperature changes. Furthermore, the voltage applied to the phase variable wave plate can be reduced, thereby providing a still more practical device.
In the fourth or sixth configuration of the optical pickup according to the present invention, it is preferable that the phase variable wave plate includes a combination of a liquid crystal layer and a film with different wavelength dispersion relationships of refractive index, the liquid crystal layer and the film are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1.
In the third to eighth configurations of the optical pickup according to the present invention, it is preferable that the plurality of coherent light sources are three coherent light sources respectively emitting light beams with the different wavelengths (xcex1 less than xcex2 less than xcex3), and the wavelengths of the three coherent light sources are in the ranges 370 nm less than xcex1 less than 430 nm, 635 nm less than xcex2 less than 690 nm and 760 nm less than xcex3 less than 810 nm.
In the fourth, sixth or seventh configuration of the optical pickup according to the present invention, it is preferable that the phase variable wave plate includes a liquid crystal layer, whose birefringence is changed by adjusting a voltage applied to the liquid crystal layer.
In the first to eighth configurations of the optical pickup according to the present invention, it is preferable that the optical pickup further includes a variable phase plate that is divided into regions of concentric rings and inserted in an optical path between the coherent light source and the optical disk, and an objective lens for focusing the light beams with the different wavelengths emitted from the coherent light sources on the optical disk. The variable phase plate compensates for spherical aberration caused by the objective lens. This preferred example can achieve excellent focusing properties.
The first configuration of the optical information recording/reproducing apparatus according to the present invention is characterized in that the optical information recording/reproducing apparatus includes an optical pickup including a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. At least one of the optical detection systems includes a polarization branching member, and the phase variable wave plate is located behind the polarization branching member. With respect to the light beams emitted from the coherent light sources with which the optical detection systems including the polarization branching member are provided, photo detectors included in the optical detection systems detect a light quantity of the reflected beams from the optical disk after passing through the polarization branching member, and a phase modulation of the phase variable wave plate is adjusted based on a detection result of the photo detectors, thereby controlling the light quantity of the reflected beams after passing through the polarization branching member. The first configuration of this optical information recording/reproducing apparatus can maintain a constant light quantity that is guided onto photo detectors included in the optical detection systems, thereby achieving a stable signal detection.
The second configuration of the optical information recording/reproducing apparatus according to the present invention is characterized in that the optical information recording/reproducing apparatus includes an optical pickup including a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. At least one of the optical detection systems includes a polarization branching member. With respect to the light beams emitted from the coherent light sources with which the optical detection systems including the polarization branching member are provided, the sum of a phase modulation of the phase variable wave plate and a phase modulation generated in the optical disk for incident path is controlled so as to be np/4, wherein n=1, 3, 5, 7, . . . With the second configuration of this optical information recording/reproducing apparatus, the reflected lights from the optical disk are converted into linearly polarized lights in which the polarization directions thereof are rotated by 90xc2x0 from the initial directions after passing through the phase variable wave plate. In other words, the phase modulation is controlled so that the largest light quantity is guided onto the photo detectors, thereby providing a servo operation and signal detection in a stable manner.
The third configuration of the optical information recording/reproducing apparatus according to the present invention is characterized in that the optical information recording/reproducing apparatus includes an optical pickup including a plurality of coherent light sources emitting light beams with different wavelengths (xcex1 less than xcex2 less than  . . . ) and with which optical detection systems for detecting a reflected beam from an optical disk are provided, and a phase variable wave plate. At least one of the optical detection systems includes a polarization branching member. With respect to the light beams emitted from the coherent light sources with which the optical detection systems including the polarization branching member are provided, the phase variable wave plate is controlled so as to function as a 1/4 wave plate. The third configuration of this optical information recording/reproducing apparatus can maintain a constant light quantity that is guided to the optical detection systems including the polarization branching member to a certain extent, thereby achieving a stable signal detection.
In the first to third configurations of the optical information recording/reproducing apparatus according to the present invention, it is preferable that the phase variable wave plate includes two liquid crystal layers with different wavelength dispersion relationships of refractive index, the two liquid crystal layers are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1.
In the first to third configurations of the optical information recording/reproducing apparatus according to the present invention, it is preferable that the phase variable wave plate includes a combination of a liquid crystal layer and a film with different wavelength dispersion relationships of refractive index, the liquid crystal layer and the film are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained by the phase variable wave plate are xcfx86n less than xcfx86n+1 for wavelengths xcexn less than xcexn+1.
In the first to third configurations of the optical information recording/reproducing apparatus according to the present invention, it is preferable that the plurality of coherent light sources are three coherent light sources respectively emitting light beams with the different wavelengths (xcex1 less than xcex2 less than xcex3), and the wavelengths of the three coherent light sources are in the ranges 370 nm less than xcex1 less than 430 nm, 635 nm less than xcex2 less than 690 nm and 760 nm less than xcex3 less than 810 nm.
In the first to third configurations of the optical information recording/reproducing apparatus according to the present invention, it is preferable that the phase variable wave plate includes a liquid crystal layer, whose birefringence is changed by adjusting a voltage applied to the liquid crystal layer.
In the first to third configurations of the optical information recording/reproducing apparatus according to the present invention, it is preferable that the optical information recording/reproducing apparatus further includes a variable phase plate that is divided into regions of concentric rings and inserted in an optical path between the coherent light source and the optical disk, and an objective lens for focusing the light beams with the different wavelengths emitted from the coherent light sources on the optical disk. The variable phase plate compensates for spherical aberration caused by the objective lens.
The first configuration of the phase variable wave plate according to the present invention is characterized in that the phase variable wave plate including two liquid crystal layers with different wavelength dispersion relationships of refractive index. The two liquid crystal layers are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained are xcfx86n less than xcfx86nxe2x88x921 for wavelengths xcexn less than xcexn+1.
The second configuration of the phase variable wave plate according to the present invention is characterized in that the phase variable wave plate including a combination of a liquid crystal layer and a film with different wavelength dispersion relationships of refractive index. The liquid crystal layer and the film are oriented perpendicularly to each other, and phase differences xcfx86 that are obtained are xcfx86n less than xcfx86nxe2x88x921 for wavelengths xcexn less than xcexn+1.